Not Applicable
Not Applicable
This invention relates to the measurement of CT numbers, and more particularly to a method and apparatus for stabilizing the measurement of CT numbers.
X-ray computed tomography (CT) allows an image of the internal structure of a target object to be generated, one cross-sectional slice at a time, by irradiating the target object with x-rays. The target object may be an anatomical region of a patient, such as the head or the chest. In this case, CT allows structures inside the human body, such as blood clots and tumors, to be visualized accurately without invasive surgery. Many other applications have been found for CT, including but not limited to, the detection of explosives in luggage and the analysis of fluids in petroleum engineering.
In a CT system, x-rays emitted from an x-ray source pass through a slice of the target object, and are detected by a detection system. The slice is irradiated from many different directions, for example by rotating the x-ray source and detector around a patient so that each revolution of the x-ray source and detector produces a scan of a single slice of the target region. In spiral (or helical) CT systems, the x-ray source rotates continuously as the patient is moved through the x-ray scan field, so that a continuous set of data is obtained for the entire region scanned.
The detection system measures the intensity of the x-ray beam that has been transmitted through a slice of the target object. The material within a slice irradiated by an x-ray beam attenuates the beam by absorbing and/or scattering the x-rays. The detection system generates detection signals indicative of the attenuated intensities of the x-rays that have traversed the slice, digitizes them, and transmits the digitized detection signals to a computer.
The computer implements image processing techniques, known in the art, to generate an image of the target object, slice by slice. Each slice is viewed as being composed of a plurality of individual volume elements. Information regarding the total attenuation of each of a very large number of x-ray beams, which traverse the patient in essentially all directions and at all radii from the center of the field of view, is used to determine the density and structure of each volume element. Each volume element is characterized by a numerical value, referred to as the CT number, which represents the x-ray attenuation characteristics of the element. CT numbers are conventionally scaled relative to the x-ray attenuation coefficient of pure water, which is assigned a CT number equal to 0 under the Hounsfield scale that ranges from low density (about xe2x88x921000) to high density (about +3095). The CT number of a material thus represents the attenuation coefficient of the material relative to the attenuation coefficient (0) of pure water. Soft tissues commonly have CT numbers in the range from about xe2x88x92100 to about 200. The CT number for bone is from about 800 to about 1400, whereas metals often have CT numbers in excess of 2000.
A CT image is generated as a map or distribution within the target object of such arrays of CT numbers. The reconstruction of an image by a CT system thus requires an accurate measurement of x-ray attenuations, and an accurate determination of CT numbers. Many industrial applications of CT require that the CT number of a material be determined with great precision and accuracy. As one example, part of the recognition algorithm used in CT scanning for the detection of explosives refers to the CT numbers of known explosives in order to recognize explosives within an object, such as luggage.
The calculated CT numbers are related to the energy of the x-ray beam, which in turn is related to the voltage provided to the x-ray source that generates the x-rays. The calculated CT numbers thus depend strongly on the voltage provided by the power supply to the x-ray source. The only other parameters that affect the CT number are those that define the physical geometry of the CT scanner and its inherent x-ray absorption, which can be maintained constant over long periods of time. Any fluctuations in the x-ray source voltage thus impair the accuracy and stability of CT number measurements by a CT system. These fluctuations are typically caused by voltage source drift, for example due to the drift of resistor values with time. A CT system must therefore have a very stable voltage source connected to the x-ray source, in order for accurate and stable measurements of CT numbers to be possible. In order to reliably make about 0.1% measurements of CT numbers for many years, the voltage provided to the x-ray source must be stable to about 0.03%, for many years.
Stability in x-ray source voltage is very difficult and expensive to obtain in practice, because very stable electronic components are typically not suitable for high voltage applications, such as the generation of x-rays. Indeed, the task of testing the electronic components of a CT system, and proving that they are stable over time and over possible changes in the environment, is very expensive and difficult. It is therefore desirable to provide a CT system which has inherently stable components that do not require such testing.
It is an object of this invention to provide a low-cost method and apparatus for allowing a CT system to determine CT numbers with improved accuracy and stability. It is another object to provide a method and apparatus for maintaining the determination of CT numbers by a CT system stable against fluctuations in the voltage provided to the x-ray source.
The invention relates to a method and apparatus for stabilizing the measurement of CT numbers against fluctuations in the x-ray source voltage. In the present invention, CT number measurements are stabilized by detecting the fluctuations in the x-ray source voltage or equivalently, in the x-ray energy, using x-ray intensity magnitudes that are measured by a device, henceforth called a kV meter.
A CT system in accord with the present invention includes an x-ray source for emitting x-rays in response to an accelerating voltage provided by a voltage source. A detection system, preferably including an array of detectors, detects x-rays emitted by the x-ray source and transmitted through a target object. The CT system further includes an image processor for reconstructing a CT image of the target object. The image processor calculates the CT numbers of the target object from the measurements of the attenuated intensities of x-rays transmitted through the target object and detected by the detector system.
The CT system also includes a kV meter. In the present invention, the kV meter includes a principal detector and an auxiliary detector. The auxiliary detector is covered with an absorber that removes a large fraction of the lower-energy x-ray photons in the incident x-ray beam. The ratio between a first x-ray intensity magnitude measured by the principal detector, and a second x-ray intensity magnitude measured by the auxiliary detector, is a strong, stable function of the voltage supplied to the x-ray source. This ratio, or the corresponding voltage, serves as a reference level for the stabilization of the power source for subsequent CT number measurements by the CT system.
As a calibration, the CT system calculates the CT number of a sample having a known CT number value. The sample may be a vessel containing water, by way of example. The voltage supplied by the voltage source is adjusted to the value which yields the correct, known CT number value for the sample. The kV meter measures the x-ray intensities from the principal and the auxiliary detector, and determines from them the magnitude of the voltage supplied to the x-ray source, when the CT number of the sample, as calculated by the CT system, matches the known CT number.
The CT system includes a controller, which provides a voltage control signal to the voltage source. The controller continuously adjusts the voltage control signal so as to maintain the voltage constant at the reference level established during calibration. Equivalently, the controller adaptively adjusts the voltage control signal based on a known and measurable function of the first and second intensity magnitudes generated by the kV meter, such as the measured ratio of the first and second intensity magnitudes as referred to above. In a preferred embodiment of the invention, the controller uses the measured ratio or other known function as a sensitive way of detecting any fluctuation in voltage from the reference level, and adaptively adjusts the control signal based on the measured ratio or other known function of the intensity magnitudes. Undesirable fluctuations in voltage are thereby prevented, and the measurement of CT numbers is stabilized against such fluctuations.
A method is provided for stabilizing the measurement of CT numbers by a CT system having an x-ray source for emitting x-rays in response to a voltage supplied by a voltage source. The method includes calibrating the CT system, initially. During calibration, the CT number of a sample (for example a water sample) having a known CT number value is calculated. The method includes adjusting the x-ray source voltage of the CT system, until the calculated value of the CT number of the sample matches the known CT number value. The magnitude of this x-ray source voltage then serves as a reference level for feedback control.
The method includes using the x-ray spectrum measured by a kV meter to keep the voltage constant against fluctuations from the reference level, caused by voltage source drift. The method includes using a principal detector, and an auxiliary detector including an absorber that preferentially absorbs x-ray photons having a relatively low energy. The method includes measuring a first x-ray intensity magnitude using the principal detector, and measuring a second x-ray intensity magnitude using the auxiliary detector.
The method includes providing a voltage control signal to the voltage source, during subsequent CT number measurements by the CT system. The method includes continually adjusting the voltage control signal as a function of the ratio (or other measurable relationship) between the first and second intensity magnitudes. In this way, the x-ray source voltage is maintained substantially constant at the reference level established during calibration, thereby substantially reducing in the CT system a variation in the measured values of CT numbers caused by voltage source drift.